Recently, as markets on watches, notebook PCs, cellular phones, televisions and monitors have been tremendously extended, the demand for displays of low weight and low power consumption has been greatly increased year after year. Especially, a liquid crystal display (LCD), which has properties such as a light weight, and thin thickness, and requires low power consumption, has been widely applied to such products.
However, an LCD has a disadvantage of viewing angle dependency. In other words, an LCD shows variations in color or light/darkness depending on viewing directions or angles. Also, as a size of a display screen increase, a viewing angle decreases. As compared to a conventional CRT (cathode ray tube) device having a viewing angle of about 180°, a TFT-LCD with no viewing angle compensation shows a viewing angle of merely about ±50°.
In order to solve the above described problems, various methods have been used, such methods including a multi-domain method in which a pixel is divided into several regions within a liquid crystal cell to control liquid crystal alignment, a method of controlling voltage, and a method of utilizing an optical compensation film.
The above-mentioned viewing angle dependency of an LCD is caused by the incident light having a tilt angle to an LCD panel, which shows a birefringence effect different from that of the vertical incident light. In order to compensate for this, a method of using an optical compensation film has been widely used, in which retardation films having opposite birefringence indexes to a panel are attached onto both surfaces of the panel. Also, as the size of a display panel has increased, a high-quality liquid crystal compensation film has been needed.
In an IPS (In-plane switching) mode where liquid crystal molecules are horizontally aligned, when two electrodes on one side substrate are driven, the liquid crystal molecules are rotated on a plane, thereby transmitting or blocking light. In such an IPS mode, the amount of light is adjusted by horizontal movement of the liquid crystal molecules, and thus the viewing angle property is significantly improved. Accordingly, the IPS mode is currently used for various products. Unlike a VA mode, the IPS mode is excellent in a viewing angle and a color shift, even in a state where a retardation film is not basically applied. However, as performance of an LCD TV is required to be improved, properties of the IPS mode, such as a viewing angle and a color shift, are required to be improved.
Recently, U.S. Pat. No. 6,115,095 disclosed a structure attached with a +C plate, that is, a positive uniaxial optical anisotropic film whose optical axis is perpendicular to the plane thereof, and a structure attached with a +A plate, that is, a positive uniaxial optical anisotropic film whose optical axis is parallel to the plane thereof. In particular, it was disclosed that when such retardation films are applied in an IPS mode, it is possible to achieve more desirable properties in a viewing angle and a color shift.
Japanese Laid-Open Patent No. 2001-166133 disclosed a technology of manufacturing a retardation film by controlling a phase difference property in an appropriate way, such as an extension treatment of a film made of a polymer, or a refractive index control of the film in a thickness direction, etc. Especially, the control of the refractive index in a thickness direction is achieved by adhering one or two heat shrinkable films to one or both surfaces of a film through an adhesive layer, etc., adding the contraction force of the heat shrinkable film to the film through heating, and thus extending or contracting the film in a vertical direction or in one side or both directions under operation of the contraction force. However, such a method has a problem in that there is a limitation in extending the film in a thickness direction, and it is impossible to control a phase difference in a thickness direction over a wide range. Also, in the method of contracting the film by adding the contraction force of the heat shrinkable film to the film, as the thickness of the retardation film is increased up to a range of about 50 to 100 microns, it is difficult to sufficiently satisfy the requirement for reduction in thickness of the film.
Also, Japanese Laid-Open Patent No. 2003-149441 disclosed a retardation film capable of controlling a phase difference in a thickness direction over a wide range, in which a C plate (a homeotropic alignment liquid crystal film) and an A plate having a retardation film are laminated and integrated, the homeotropic alignment liquid crystal film being formed by a homeotropic alignment liquid crystal composition containing a homeotropic alignment side-chain type liquid crystal polymer or a photopolymerizable liquid crystal compound together with the side-chain type liquid crystal polymer. However, the method of forming a homeotropic alignment liquid crystal film by using a side-chain type liquid crystal polymer has a problem in that a high temperature heat treatment with a glass transition temperature (Tg) (that is, a heat treatment temperature in which the polymer can be in a liquid crystal phase) of 60˜300° C., (more specifically, of 70˜200° C.) is required, and is difficult to apply to a continuous process at a high speed because the heat treatment requires 20 seconds to 30 minutes. Also, when the heat treatment is completed, a cooling treatment, such as air cooling or water cooling, etc. is required in order to fix the alignment of the homeotropic alignment liquid crystal film, and thus it is more difficult to apply the method to the continuous process. Also, the liquid crystal polymer containing liquid crystal polymers has low solubility in a conventionally used solvent.
Methods of manufacturing an optical film using a polymerizable liquid crystal have been recently reported in many reports. However, in most of them, an optical film is manufactured by coating a polymerizable liquid crystal material on a substrate of an alignment film, aligning the coated material, and fixing the aligned material through exposure, but a polymerizable liquid crystal material for a homeotropic alignment is not specifically mentioned.